practicing is a present participle, practice is an infinitive. You will be learning the differences between the two in future Duolingo lessons. Basically, present participles indicate an action that is happening right now, and infinitives are used when discussing the word itself and not the action it describes.

"I think it is..."

You can't remove the subject of the verb like you can in Spanish, it is always required in English.

"My dad said I have to know English" or "My dad told me I have to know English"

You have to say "My dad said I have to know English" instead because the verb told requires an object in this context.

"So if you want to test me"

test is an infinitive here, and infinitives require the word to before them. When a verb comes before another verb (in this case want comes before test), the second verb has to be an infinitive.